The methods for introducing transgenes in plants by Agrobacterium mediated transformation utilizes a T-DNA (transfer DNA) that incorporates the genetic elements of the transgene and transfers those genetic elements into the genome of a plant. Generally, the transgene(s) bordered by a right border DNA molecule (RB) and a left border DNA molecule (LB) is transferred into the plant genome at a single locus. It has been previously observed that when a DNA construct contains more than one T-DNA these T-DNAs and the transgenes contained within may be integrated into the plant genome at separate loci and is referred to a co-transformation (U.S. Pat. No. 5,731,179, WO 00/18939). The process of co-transformation, where two T-DNAs are at different loci in the plant genome and therefore segregate independently in the progeny, can be achieved by delivery of the T-DNAs with a mixture of Agrobacteria transformed with plasmids carrying the separate T-DNA (Depicker et al., Mol. Gen. Genet. 201:477-484,1985; Petit et al., Mol. Gen. Genet. 202:388-393, 1986; McKnight et al., Plant Mol Biol. 8:439-445, 1987; DeBlock et al., Theor. Appl. Genet. 82:257-263, 1991; Komari et al., Plant J. 10:165-174,1996; De Neve et al., Plant J. 11:15-29, 1997; Poirier et al., Theor. Appl. Genet. 100:487-493, 2000, herein incorporated by reference in their entirety). Co-transformation can also be achieved by transforming one Agrobacterium strain with two binary DNA constructs, each containing one T-DNA (Daley et al., Plant Cell Reports 17:489-496 1998). An additional method employs constructing the two T-DNAs on a single DNA vector, transforming the vector into a plant cell and then identifying the transgenic cells or plants that have integrated the T-DNAs at different loci (U.S. Pat. No. 5,731,179, WO 00/18939, Komari et al., Plant J. 10:165-174, 1996; Xing et al., In Vitro Cell Devel. Biol. Plant 36:456-463, 2000).
Unlinking T-DNAs that contain different plant expression cassettes can be useful in the development of transgenic plants, especially if one of the plant expression cassettes does not contribute to an agronomically useful trait introduced into the crop plant. It has been previously observed that Agrobacterium Ti plasmid T-DNAs can insert into the genome of a transformed plant at more than one loci and will segregate in the progeny of the plant (Framond et al., Mol. Gen. Genet. 202:125-131, 1986). The identification of unnecessary or unwanted transgene DNA in transformed plants has been the subject of numerous investigations and many different methods have been examined in efforts to eliminate these transgenes or vector DNA from the plants (Hanson et al., Plant J. 19:727-734, 1999; Dale et al., Proc. Natl. Acad. Sci. 88:10558-10562, 1991; Ebinuma et al., Proc. Natl. Acad. Sci. 94:2117-2121, 1997; Yoder et al., Bio/Technology 12:263-268, 1994; Kononov et al., Plant J. 11:945-957, 1997).
A two T-DNA system is a useful method to segregate the marker gene from the agronomically important gene of interest (GOI) in a transgenic plant. The marker gene generally has no further utility after it has been used to select or score for the transformed plant cell. The research effort to develop an efficient two T-DNA system that provides separate and distinct genomic integration sites has been extensive. A single DNA vector carrying the two-T-DNAs is the preferred method to construct a two T-DNA transformation system. However, because of the occurrence of both T-DNAs on a single DNA construct often both are transferred into the plant genome at the same loci. This occurs when one of the border DNA molecule of the first T-DNA is not recognized during the integration process. This reduced efficiency adds to the cost of producing the events and selecting for the individuals that have T-DNAs integrated at an independent locus. It would be of general utility to have DNA constructs and a method where it is possible to chemically select against individuals that have incorporated the two T-DNAs at the same loci or have incorporated unnecessary DNA that occurs in the spacer region between the two T-DNAs, or unnecessary DNA that occurs in the region of the DNA construct flanking the two T-DNAs.
Heterologous dominant conditional lethal genes have the greatest potential for controlled cell lethality. The genes are by definition, non-lethal in the absence of the controlled application of a heterologous protoxin and utilize protoxins which are not substrates for normal cellular enzymes. In plants, some examples of heterologous conditional lethal genes are the pehA gene that converts a nontoxic phosphonate of glyphosate into glyphosate (U.S. Pat. No. 5,254,801, the entirety of which is herein incorporated by reference), argE gene that converts a nontoxic N-acetyl-phosphinothricin to toxic phosphinothricin (Kriete et al. Plant J. 9:809-818, 1996, the entirety of which is herein incorporated by reference), the iaaH gene encoding indoleacetamide hydrolase which can convert non-toxic levels of naphthalene acetamide into toxic levels of the auxin, naphthalene acetic acid (Klee et al., Genes Dev. 1:86-96, 1987). A bacterial cytosine deaminase gene has been shown to function as a conditional lethal gene for negative selection in plants (Kobayashi et al. Jpn. J. Genet. 70:409-422; Perera et al. Plant Mol. Biol. 23:793-799, 1993, the entirety of which is herein incorporated by reference). P450 monooxygenase expression in plant cells converts a sulphonylurea compound (R7402) with low toxicity into a more highly phytotoxic form (O'Keefe et al. Plant Physiol. 105:473-482, 1994, the entirety of which is herein incorporated by reference). Transgenic plants expressing β-glucuronidase in male tissues can convert a glucuronic acid conjugated protoxin into a phytotoxic molecule providing a gametocide (WO9204454, the entirety of which is herein incorporated by reference). Viral thymidine kinase is an example of a heterologous dominant conditional lethal gene in mammalian cells the functions in plants (Czako et al. Plant Physiol. 104:1067-1071, 1994). Unlike the cellular thymidine kinase, the viral thymidine kinase protein is able to activate pyrimidine analogs such as acycolvir and gancyclovia into toxic products. The most effective conditional lethal gene enzymes have no affect on cellular metabolism in the absence of the protoxin and utilizes a protoxin that is at least several fold less toxic than the activated toxin.
The present invention provides a conditional lethal transgene in the two T-DNA construct and a method to select against a plant cell, plant or progeny thereof that express a conditional lethal phenotype. A conditional lethal phenotype involves the expression of a gene product that is not lethal to a cell under normal conditions. To this end the conditional lethal phenotype is used to eliminate the unwanted plant cells, plant tissues or plants when it is most desirable to do so. The present invention provides DNA constructs in novel combinations with positive selectable marker transgenes and methods for the use of these DNA constructs with or without a conditional lethal phenotype in a two T-DNA system for plants. The methods of the present invention enhance the identification and selection of transgenic events in which the two T-DNA segments have integrated into the plant genome at different physical and genetic loci, and provide transgenic plants with agronomically useful transgenes and phenotypes free of the marker transgenes.